1. Field of the Invention
The present invention relates to a method of forming SOI (Semiconductor On Insulator) structure by SIMOX (Separation by Implanted Oxygen) method and particularly to a method of forming SOI structure containing less crystal defects.
Since a substrate having the SOI structure (hereinafter called a SOI substrate) has excellent merits, when devices are formed thereon, that is, a parasitic capacitance is small and high speed operation of devices can be realized, device isolation can be done easily, immunity from radiation such as alpha rays is high and a high voltage resistant device may be formed, etc., practical application into MOS IC, etc. is now often attempted.
2. Description of the Related Art
A substance formed by the SIMOX method is an example of the SOI substrate. In the SIMOX method, a silicon dioxide (SiO.sub.2) layer is formed in a silicon single crystal substrate by ion implantation method of oxygen. Usually, the oxygen ion of 80.about.200 KeV is implanted in the dose of 10.sup.17 .about.10.sup.18 cm.sup.-3 and thereafter the annealing is carried out at the temperature of about 1100.degree. C. Thereby, a neighboring layer of the surface which has changed to an amorphous layer is then single-crystallized and a SiO.sub.2 layer is formed in the region thereunder where the concentration of implanted oxygen becomes maximum in such a manner as taking also the oxygen in the vicinity of surface. The silicon single crystal layer in the vicinity of the surface corresponds to SOI and this layer is used for formation of devices such as IC, etc. The SIMOX method and application into devices are described, for example, Sorin Cristoloveanu: "Electrical Evaluation of SIMOX Material and Integrated Devices" Material Research Society Symposium Proceedings Vol. 107, p. 335-p. 347, 1988.
The SOI substrate formed by SIMOX method is hereinafter called a "SIMOX substrate" and silicon single crystal layer formed on the insulation film of "SIMOX substrate" is called the "SIMOX SOI". The manufacturing condition of the "SIMOX substrate" used actually is, for example, as follows:
The original silicon single crystal substrate is an n-type wafer having a specific resistance of 1 k.OMEGA.-cm and surface orientation of (100). The oxygen ion is implanted to the surface of this wafer in the implantation energy of 150 KeV and dose of 1.times.10.sup.18 cm.sup.-2. Under this condition, the project range of oxygen ion is 0.37 .mu.m. Thereafter, this substrate is subjected to the heat treatment for 60 min. at 1200.degree. C. in the nitrogen ambience. As a result, the SiO.sub.2 insulation film formed thereby has the thickness of about 200 nm and the silicon single crystal layer, namely "SIMOX SOI" formed thereon has the thickness of about 200 nm.
However, this "SIMOX SOI" usually has the crystal defect of 1.times.10.sup.7 cm.sup.-2. If the "SIMOX SOI" having such many crystal defects is formed, leak current increases and thereby it becomes difficult to obtain a device having good device characteristics.